The present invention relates to nuclear reactors and, more specifically, to techniques for cooling nuclear reactors, and their containments, in the event of one or more types of malfunctions.
During normal operation of a nuclear reactor, the nuclear fuel in a reactor vessel remains covered with water to generate steam. The nuclear fuel consists of fuel rods which develop substantial internal heat. After shutdown of the reactor, decay reactions continue to generate heat in the fuel rods for a lengthy period.
It is important to environmental safety to ensure that radioactive materials are not released during normal or abnormal operation. Such radioactive materials include, for example, steam generated in the reactor vessel or water that is condensed from such steam.
Abnormal operations to be discussed in the following disclosure include a loss-of-coolant accident which may occur due to a break in a component or piping such as, for example, a steam pipe within the reactor building. The three requirements in such a situation are 1) to replace water in the reactor pressure vessel to cover the fuel rods, 2) to dissipate the heat existing immediately following the break, and 3) to remove the decay heat over an extended period (days or weeks) following the break, such that structural integrity of the containment vessel is maintained.
In the prior art, the movement of cooling water to satisfy the three foregoing requirements is provided as a result of forced circulation by high-pressure water pumps driven by electricity or other external power source. In the event of failure of the normal electrical grid supplying electric power to the plant, diesel generators are provided to take over the task of supplying power for driving the pumps. It is a fact, however, that there is a small but finite probability, that diesel generators can fail to function at a critical time, or that human errors can incapacitate systems. Such failure following a serious loss of coolant accident such as, for example, a break in a steam pipe, can be considered a worst-case scenario.
An electric generator driven by steam from a nuclear reactor can experience a sudden loss of load for a number of causes. In such a situation, a concomitant reduction in demand for steam occurs at a rate that exceeds the ability of a reactor control system, and the normal power-driven cooling system, to accommodate. In the past, an isolation condenser disposed in a pool of water receives and condenses excess steam until reactor control reduces the production of heat to a value within the capability of the cooling system, or until the generator load is resumed. The pool of water is open to the atmosphere, but the steam and condensate remain in the heat exchanger, isolated from the atmosphere.
The prior art appears innocent of any teaching of the use of an isolation condenser, or similar heat exchanger, as part of a passive system for dissipating heat following a reactor operating transient or an accident.